The present invention relates to a process for modifying the surface characteristics of reactive metal and metal alloy substrates and to the surface-modified metal or metal alloy substrates produced thereby.
Reactive metals commonly used in engineering applications, such as aluminum, magnesium and titanium, are soft to scratch but serviceable in their elemental state. Most of the alloys of such reactive metals are similarly serviceable despite being soft to scratch. However, the scope of the potential usefulness of these metals and alloys would be greatly expanded if the surfaces of the metals and alloys could be rendered scratch or abrasion resistant.
The present inventor has discovered a method whereby the surfaces of reactive metals or alloys can be modified so that the surface properties of the metal or alloy are beneficially changed. In accordance with the present invention, this modification requires the use of a material such as fly ash, which is produced by the combustion of fossil fuels, or a mixture of powdered oxides similar to those contained in fly ash.
The basic chemical reactions which occur during the process of the present invention are known and occur in a process known as the "Thermit process". The Thermit process is generally practiced to allow field repair of iron or steel sections, such as railroad tracks. In the Thermit process, aluminum powders are mixed with iron oxide powders and ignited. The process is autogeneous and is terminated when the iron oxide is completely reduced by the aluminum. The aluminum is oxidized and the heat of oxidation is so high that the iron produced is in a superheated molten condition. The ratio of iron oxide to aluminum must be controlled so that an excessive amount of aluminum is not available to alloy with the iron.
It has now been discovered that it is possible to cause the chemical reaction between aluminum and fly ash or other oxides having a heat of oxidation lower than that of aluminum oxide, at temperatures below the melting point of aluminum. The inventive process provides a means for generating a tightly bonded abrasion resistant layer on a metal or metal alloy substrate by preparing the surface of a metal or metal alloy substrate, packing it in fly ash or a mixture of iron oxide and other components, and then heating it to temperatures just below the melting point of the metal or alloy, thereby producing a tightly bonded surface layer on the metal or alloy substrate. Since the surface layer is tightly bonded to the substrate, it is also possible to build up a layered structure by providing a plurality of metal or alloy substrates, packing fly ash or a mixture of oxides including iron oxide between the substrates, and then heating the layered structure to just below the melting point of the metal or alloy. The resulting structure comprises alternate layers of a very soft ductile material and a very hard brittle material. The resultant product is macroscopically reminiscent of the steel microconstituent pearlite or of the macrostructure of Damascus steel.
It has further been discovered that a hard grinding material can be made without employing the high temperatures required to make alundum or synthetic corundum from bauxite. In accordance with a known process, artifical corundum or emery is manufactured by causing iron oxide and aluminum oxide to fuse together. This operation is carried out by heating the mineral bauxite at very high temperatures, above 2,000.degree. F., and results in an industrially useful grinding material known as alundum. According to one aspect of the present invention, a hard grinding material can be made by combining fly ash, or a similar mixture of oxides and powdered aluminum and then heating the mixture at a temperature well below 2,000.degree. F. The hard grinding material of this invention can be bonded in situ during the production thereof to a structural tool part or can be shaped into a suitable configuration for later use as an insert for a machining operation.